The Centers for Disease Control and Prevention (CDC) estimates that at least two million illnesses and 23,000 deaths annually are caused by antimicrobial-resistant bacteria in the United States. The Gram-negative (G-) pathogens are of particular concern, as they account for roughly 99,000 deaths and $20B in health care costs a year. More alarming, treatment options for G- infections have become increasingly limited due to rapid emergence of multi-drug resistance (MDR) to existing and newly approved antimicrobial agents, highlighting the need for alternative strategies to prevent MDR G- infections. Thus, an agent that leverages immunological mechanisms to prevent infection in high risk populations from drug susceptible and MDR strains would possess a unique advantage in addressing this need. The innovative Cloudbreak? Antibody Drug Conjugates (ADCs) platform, developed at Cidara Therapeutics, uses a fundamentally new immune-based approach to prevent and treat G- infections. Similar to successful cancer bispecific agents, ADCs bind conserved targets on pathogens via a Targeting Moiety (TM) while simultaneously engaging multiple arms of the immune system via an Effector Moiety (EM). The TM is comprised of a dimeric peptide that binds tightly to lipopolysaccharide (LPS) and confers broad spectrum G- coverage with potent intrinsic antimicrobial activity. The EM is a human IgG1 Fc, which collectively activates complement dependent cytotoxicity (CDC), antibody (Ab)-dependent cell-mediated cytotoxicity (ADCC), and Ab-dependent cell phagocytosis (ADCP) to clear MDR G- pathogens from the host, via recognition by Fc? receptors on host cells.!This innovative approach involving efficient cell targeting with inherent cell killing catalyzes a robust immune response by more effectively presenting the pathogen to immune components for clearance. CTC-026 is our lead ADC candidate and has demonstrated highly promising properties as an immunoprophylactic agent: broad spectrum antibacterial activity that is both intrinsic and immune-driven, acute safety in rodents, in vivo efficacy in mouse models of Escherichia coli sepsis and Acinetobacter baumannii pneumonia, and a 67 hour plasma half-life in mice. Further optimization of potency and spectrum and in-depth evaluation of pharmacological and toxicological properties of this lead are proposed in this application. The overarching goal of this proposal is to identify a qualified lead development candidate in Year 3 and an Investigational new drug (IND) candidate by the end of Year 5, that meets these criteria: 1) acceptable stability and solubility for IV formulation, 2) MIC90s ?1 M against clinical isolates (including MDR) of Klebsiella, Acinetobacter, Pseudomonas and E. coli, 3) MIC90s ?1 M against MCR-1, MCR-2 and other colistin- resistant G- clinical isolates, 4) robust in vivo prophylactic efficacy against MDR G- infections in a time window 48-72h prior to infection, 5) PK/PD parameters to support once weekly or better dosing in humans, 6) a NOAEL in GLP toxicology studies in rats and Cynomolgus monkeys at least fivefold higher than the targeted clinical dose, and 7) a scalable synthesis to GMP product.